This invention pertains to respirator apparatus and methods for operating the same, and particularly to such apparatus and operating methods which feature intermittent demand oxygen flow and/or apneic event detection.
Intermittent demand oxygen flow has been tried in the past as an attempt to reduce high costs generally involved in supplying oxygen flow to a patient. D. Auerbach et al. (Chest, 74: 1 July 1978, pp. 39-44) briefly review the history of such attempts and report test results observed with an oxygen cannula system using intermittent-demand nasal flow. The device reported by Auerbach et al. uses a spring-loaded diaphragm in conjunction with a cannula to sense, in two distinct modes, either the negative pressure created by inspiration or the positive pressure created by expiration. In a negative mode the reported device supplies oxygen to the patient as long as negative pressure is detected; in the positive mode oxygen is supplied as long as a positive pressure is not detected.
Fluidic logic elements have also been used in intermittent demand oxygen systems to sense negative and positive pressures created by inspiration and expiration. In this regard, U.S. Pat. No. 3,976,065 to Durkan discusses prior art ventilators employing fluidic elements and further discloses a digital fluidic ventilator wherein a single fluidic flip-flop serves as a primary control element and wherein any one of a plurality of operating modes is obtained by adjusting a bias signal applied to the flip-flop.
Prior art intermittent demand oxygen devices, whether sensing negative or positive pressure, supply oxygen to a patient substantially throughout the duration of an inspiration. If a given patient were to breathe at the rate of 10 breaths per minute, for example, each breath would average 6 seconds. For such a patient, an inspiration would normally be sensed for about 2 seconds of the 6 seconds per breath; an expiration would normally be sensed for the four remaining seconds. Prior art devices, therefore, would supply oxygen for the full duration of inspiration--in this case, 2 seconds.
In some existing devices, such as that tested by Auerbach et al., supra, the oxygen supplied for the full duration of inspiration tends to commence with a surge in the pattern of flow. Heretofore this surge has been considered unnecessary and wasteful. In some devices other fluidic elements, such as a flowmeter, for example, have been incorporated intermediate the patient and the oxygen supply to dampen the surge.
The applicant has clinically observed, however, that in the breathing process oxygen is absorbed into the blood essentially only during an early stage of inspiration. That is, it is during an early stage of inspiration that oxygen effectively reaches the alveoli. Oxygen applied during the latter stages of inspiration remains in "dead spaces" such as the pharynx, trachea, and bronchial tubes. Hence, the applicant has observed and concluded that in operating respirator apparatus it is more advantageous to apply a greater volume of oxygen per second and to apply the oxygen only during an effective early stage of inspiration rather than to apply a conventional volume per second throughout the duration of inspiration.
For the case described above, the effective early stage may last for approximately 0.25 seconds. For most cases, the effective early stage is less than approximately one-quarter and usually approximately one-eighth of the duration of the inspiration. Therefore, if oxygen were supplied at twice the normal volume per second rate (for example: 100 cc/sec. rather than 50 cc/sec.), a savings of more than one-half--and in most instances more than three-quarters--would be realized. Present day intermittent demand oxygen devices are not capable of operating in accordance with this effective early stage inspiratory phenomenon.
Various prior art respirator apparatus attempt to detect apneic events. Basically, apnea is a breathing disorder that may be caused by cessation of central nervous system output, by upper airway obstruction, or by a combination of the two. This disorder has been implicated in the Sudden Infant Death Syndrome. The condition is also especially hazardous in patients with chronic obstructive lung disease since dangerous cardiac arrhythmias can occur due to the anoxia.
Of the prior art devices that attempt to detect apneic events, many (including the devices disclosed in U.S. Pat. Nos. 3,357,428 to Carlson and 4,206,754 to Cox) periodically generate electrical (as opposed to fluidic) signals which are electrically monitored to determine when a patient has ceased to breathe satisfactorily and which activate an alarm as a warning indicator.
Prior art respirators which utilize fluidic signals to detect apneic events basically employ fixed capacitance reservoirs for either gauging or controlling the length of time between inspirations. Examples of such respirators are seen in U.S. Pat. Nos. 3,910,270 to Stewart; 3,659,598 to Peters et al.; and, 4,141,354 to Ismach. In apneic event detectors using fluidic signals, however, fixed volume capacitances are inadequate since fluid compression in a fixed volume is not compatible with the low pressures often used in a fluidic logic circuit. Moreover, the prior art fixed volume capacitances exhaust through fluidic logic devices themselves and at a rate much slower than what is desirable in an efficient apneic detector.
In view of the above, an object of this invention is to provide a respirator apparatus and method of operating the same wherein respirating gas is supplied to a patient substantially at the beginning of an inspiration and for a time period thereafter which is a fraction of the duration of inspiration.
An advantage of this invention is the provision of an economical and efficient respirator apparatus allowing the conservation of a respirating gas, such as oxygen.
Another object of this invention is to provide a respirator apparatus having an fluidically-operated apneic event detector.
A further advantage of this invention is the provision of a respirator apparatus having an apneic event detector with a variable volume capacitance compatible with fluidic logic elements and means for rapid exhaust thereof.